Recent work from our laboratory suggests that blood coagulation is directly involved in the development of a specific platelet aggregation which occurs in association with the polymerization phase of fibrin clotting. The proposed study is intended to investigate the interaction between platelets, fibrinogen and polymerizing fibrin. The formation of foluble fibrin on platelets will be studied by labeling normal and thrombin-treated platelets with C14-glycine ethyl ester, followed by determination of incorporation of radioactive labels in platelet constituents employing platelet fractionation and sodium dodecylsulfate polyacrylamide gel electrophoresis techniques. The adsorption of I125-fibrinogen and I125-soluble fibrin will be investigated in normal and activated platelets. After activation by various agents, platelets will be separated by gel filtration and by albumin density gradient techniques and their ability to adsorb fibrinogen and to interact with polymerizing fibrin will be studied. Adsorbed platelet surface fibrinogen will be identified ultrastructurally by autoradiography and by the ferritin-conjugated antibody technique. The difference in platelet membrane morphology before and after the adsorption of soluble fibrin and during different phases of thrombin-induced aggregation will be investigated by the freeze cleaving and etching procedure.